An inkjet printer covers a broad range of intended purposes such as personal use, office use, business use, recording use, color display use, and color photo use as a result of its high functionalization. Moreover, in order to correspond to speed enhancement and improvement in image quality, the advancement has been made to make ejected droplets (ink droplets) fine by improving devices. In order to make the ejected droplets fine, it is necessary to micronize a pigment (particle) in an ink and finely disperse the micronized pigment in a dispersion medium.
The clarity, color brightness, and color density of a printed matter are being improved by using an ink comprising a micronized pigment. Particularly, chroma being a color value to be an index to show the color brightness is improved by using an ink comprising a micronized pigment. Moreover, a gloss value is improved in the case where the recording is carried out on processed paper for inkjet (such as photo paper and paper for wide format printing). However, since the ink comprising a micronized pigment becomes liable to penetrate into the paper, the color developing property being a color density tends to be lowered.
Accordingly, studies on improving print density by suppressing the penetrability of an aqueous pigment ink, suppressing the wetting of the ink to the paper surface, and keeping the micronized pigment near the paper surface have been made in recent years. For example, attempts to coat (encapsulate) a pigment using a graft polymer or block polymer comprising a highly hydrophilic site and a highly hydrophobic site have been made. Since the pigment is coated by allowing the highly hydrophobic site to be adsorbed on the pigment surface, the penetration of the ink into the paper is suppressed and the pigment tends to be present on the paper surface. Moreover, since the highly hydrophilic site has affinity to a dispersion medium, the graft polymer or block polymer is supposed to take an elongated form. Therefore, it is considered that the storability of the ink is improved due to electrostatic repulsive force and repulsive force by steric hindrance (see Patent Literatures 1 to 3).